Method of pre-stressing a tendon

ABSTRACT

A tendon  10  is installed in a hole  34  in a mine hanging wall  36  by making use of a barrel  14  and a wedge arrangement  16 . One end of the tendon is anchored in the hole  34  such that the other end protrudes therefrom. The barrel  14  is positioned over the protruding end of the tendon  10 . A tensioning jack applies a tensile load to the tendon  10  and an oppositely disposed load to a shear ring  20  of the barrel. When the desired preload is reached the shear ring  20  fails permitting the tensioning jack  40  to displace wedge elements  30  of the wedge arrangement  16  inwardly thereby locking the barrel on the tendon and maintaining the desired preload in the tendon.

FIELD OF THE INVENTION

THIS INVENTION relates to the tensioning of tendons. More particularlyit relates to a method of pre-stressing a tendon. It also relates to abarrel, to a pre-stressing kit and to a setting tool suitable for use inthe method.

SUMMARY OF THE INVENTION

The Inventor is aware that pre-stressed or pretensioned tendons, e.g.lengths of wire rope or steel rods, are often used in the constructionand mining industries. For example, in the mining industry, tendons,e.g. in the form of lengths of wire rope, are often used for support,e.g. for the hanging wall, in an underground mine. This can be achievedby drilling a hole in the hanging wall and anchoring an end of thetendon in the hole, e.g. adhesively or mechanically. A bearing element,e.g. an apertured plate, is positioned over the end portion of thetendon protruding from the hole in the hanging wall. The tendon may betensioned using a barrel and wedge system. This entails positioning abarrel having a hole therethrough over the end portion of the tendon inabutment with the bearing element. The tendon is tensioned to apredetermined level and the barrel is secured to the tensioned tendon bya wedge arrangement positioned between the barrel and the tendon. Hence,the tension in the tendon urges the barrel against the bearing elementwhich in turn is urged into abutment with the hanging wall to supportthe hanging wall.

In order to install the barrel, use is made of a tensioning jack. Thetensioning jack includes a nose which abuts the barrel, a grippingformation which grips the protruding end of the tendon and displacementmeans for displacing the gripping formation and nose in oppositedirections to tension the tendon.

One problem associated with this system is that difficulty can beencountered in setting or blocking of the wedge, i.e. in urging thewedge between the barrel and the tensioned tendon. In addition, onceinstalled there is no indication that the tendon has been correctlytensioned.

It is an object of this invention to provide means which the Inventorbelieves will at least alleviate this problem.

According to one aspect of the invention there is provided a method ofpre-stressing a tendon using a barrel and wedge system which includesthe steps of

positioning a barrel having a hole therethrough over the tendon;

applying a tensile load to the tendon; and

applying via a setting member an oppositely disposed force to a spacingmember which is configured to transmit said oppositely disposed force tothe barrel until a predetermined force is achieved at which stage thesetting member urges the wedge arrangement between the barrel and thetendon to secure the barrel in position on the tendon.

The method may include the step of positioning at least part of thewedge arrangement in the hole such that it is a loose fit between thebarrel and the tendon prior to applying a load to the tendon, thespacing member being configured to maintain a clearance between thesetting member and the wedge arrangement until the predetermined forceis achieved at which stage the spacing member permits the setting memberto engage the wedge arrangement and urge it between the barrel and thetendon.

The spacing member may fail structurally at the predetermined load whichresults in the tendon being tensioned to the desired tension. Preferablythe structural failure of the spacing member at the predetermined loadwill be sudden or catastrophic thereby allowing the degree ofpretensioning to be controlled fairly accurately.

The spacing member may fail in shear.

According to another aspect of the invention there is provided a barrelfor use in pre-stressing a tendon, which includes

a body having a hole therethrough; and

a spacing member connected to the body and configured to transmit a loadfrom a setting member to the body.

The spacing member may be configured to fail structurally when a load inexcess of a predetermined load is applied thereto.

The body may be generally circular cylindrical in shape, the holeextending axially therethrough and having a diameter which decreasesover at least part of its length towards a leading end of the body. Theleading end of the body may be rounded.

The spacing member may be annular in form.

The spacing member may be in the form of a shear ring.

An annular recess may be provided in the body immediately adjacent theshear ring into which recess at least part of the material of the shearring is displaced, in use, when the shear ring fails.

The barrel may include indicating means to indicate that the spacingmember has failed and hence that a tendon has been tensioned orpre-stressed to the required tension. The indicating means may include afiller material in the recess which filler material is at leastpartially displaced when the shear ring fails. The filler material maybe configured at least partially to absorb shock loads when the shearring shears. In one embodiment of the invention, the filler material maybe an epoxy. In another embodiment of the invention, the filler materialis a metal such as a aluminum. In yet another embodiment of theinvention, the filler material is a synthetic plastics material.

The shear ring may be formed integrally with the body. In a preferredembodiment of the invention, the shear ring may be defined between apair of axially spaced apart annular recesses in the body.

According to yet another aspect of the invention there is provided apre-stressing kit which includes

a barrel as hereinbefore described; and

a wedge arrangement, at least part of which is receivable, in use, inthe hole in the body between the body of the barrel and a tendonextending therethrough.

According to still yet another aspect of the invention there is provideda pre-stressing kit which includes

a barrel having a body through which a hole extends;

a wedge arrangement, at least,part of which is receivable, in use, inthe hole in the body of the barrel between the body and a tendonextending therethrough; and

a spacing member which is configured, in use, to transmit a force from asetting member to the body of the barrel until a predetermined force isreached at which stage the spacing member permits the setting member tourge the wedge arrangement between the body of the barrel and the tendonto secure the barrel in position on the tendon.

The spacing member may be formed integrally with the body.

The wedge arrangement may include a plurality of wedge elements.

According to a further aspect of the invention there is provided asetting device for use in pre-stressing a tendon using a barrel andwedge system, the barrel of which includes a body and a spacing memberin the form of a shear ring fast with the body, the setting deviceincluding

a nose having a leading end and defining an annular radially inwardlyopen channel-shaped formation adjacent the leading end; and

a shear ring engaging member which is floatingly mounted and heldcaptive in the channel-shaped formation and which is configured toengage and transmit a load to the shear ring of a barrel.

The shear ring engaging member may be in the form of a discontinuousring.

The nose and the discontinuous ring may include complementary formationsconfigured to cause the discontinuous ring to contract diametricallywhen the discontinuous ring and nose are urged towards one another inuse.

The setting device may be in the form of a tensioning jack and includesa gripping formation for gripping a tendon and displacement means fordisplacing the gripping formation and the nose in opposite directions.

The invention will now be described, by way of example, with referenceto the accompanying diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 shows a tendon installed in the hanging wall of a mine inaccordance with the invention;

FIGS. 2 and 3 show, on an enlarged scale, sequential steps involved inthe installation of the tendon of FIG. 1;

FIG. 4 shows an axial sectional view of part of a setting device inaccordance with the invention; and

FIG. 5 shows a three-dimensional view of a barrel used in the tendoninstallation of FIGS. 1 to 3.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, reference numeral 10 refers generally to a pretensionedtendon installed in accordance with the invention.

In order to install the tendon 10, use is made of a pretensioning kit,generally indicated by reference numeral 12. The pretensioning kit 12includes a barrel 14 and a wedge arrangement 16.

As can best be seen in FIG. 5 of the drawings, the barrel 14 includes abody 18. A spacing member, generally indicated by reference numeral 20is connected to the body 18.

The body 18 is generally circular cylindrical in shape and has a partspherical leading end 18.1. A hole 22 extends axially through the body18, the diameter of the hole 22 decreasing towards the leading end 18.1.

The spacing member 20 is in the form of a shear ring which is definedbetween a pair of annular recesses 24, 26. Hence, the shear ring 20 isformed integrally with the body 18. A filler material 28 is contained inthe recess 26. The filler material 28 will typically be of a relativelysoft material such as an epoxy, a synthetic plastics material, aluminumor the like.

The wedge arrangement 16 includes three wedge elements 30 at least partsof which are receivable in the hole 22 between the radially innersurface of the body 18 and a radially outer surface of the tendon 10.The radially inner surfaces of the wedge elements 30 are provided withteeth-like gripping formations 32 configured to grip the outer surfaceof the tendon 10 as described in more detail herebelow.

In use, in order to install the tendon 10, a hole 34 is drilled in ahanging wall 36 in a mine. One end of the tendon 10 (not shown) isanchored in the hole 34 e.g. adhesively or mechanically in aconventional fashion such that the tendon 10 protrudes from the hole.

A bearing element in the form of an apertured plate 38 is positionedover the protruding end of the tendon 10. A barrel 14 is then positionedover the protruding end of the tendon such that the rounded leading end18.1 thereof is in abutment with the plate 38. The wedge elements 30 ofthe wedge arrangement 16 are then positioned between the radially innersurface of the body 18 and the tendon 10 such that they are a loose fittherein.

Use is then made of a tensioning jack to tension the tendon 10. Thetensioning jack includes a nose 40 (part of which is shown in FIGS. 2and 3 of the drawings) which is configured to engage the barrel 14 asdescribed in more detail herebelow. The tensioning jack further includesa gripping formation 80 for gripping the protruding end of the tendon 10and displacement means 82 for displacing the nose 40 and the grippingformation in opposite directions.

As can best be seen in FIG. 2 of the drawings, the nose 40 has a leadingend 40.1 from which an annular shoulder 42 protrudes axially. Theshoulder 42 has an inner diameter which is slightly larger than thediameter of the trailing end of the body 1 8 such that the shoulder 42is receivable in the recess 24 in abutment with the shear ring 20. Thelength of the shoulder 42 is selected such that when the shoulder 42 isin abutment with the shear ring 20, the leading end 40.1 of the nose 40is spaced from the trailing end of the body 18. Hence, the wedgearrangement 16 is held captive and with clearance between the barrel 14,tendon 10 and leading end 40.1 of the nose 40.

The displacement means 82 is then operated to displace the tendongripping formation 80 and the nose 40 in opposite directions so that atensile load is applied to the tendon and an equal and oppositelydisposed force is applied to the shear ring 20 and hence to the body 18.When the load reaches a predetermined level, the shear ring 20 shearspermitting displacement of the nose 40 in the direction of arrow 44 sothat the leading end 40.1 of the nose 40 abuts the wedge arrangement 16and drives it between the barrel 14 and the tendon 10 as shown in FIG. 3of the drawings. As the wedge arrangement 16 is driven in the directionof arrow 44 relative to the barrel 14, the complementary taper on theradially inner surface of the body 18 and on the wedge elements 30causes the wedge elements to move radially inwardly and grip the tendon10. Hence, contraction of the tendon 10 is resisted and the tensilepreload is maintained.

It is to be appreciated, that the shear ring 20 will fail rapidly andthat the material of the shear ring will be displaced into the recess 26thereby displacing at least some of the filler material 28. This thenserves as an indication that the tendon 10 has been correctly installedand pretensioned.

Once the tendon 10 has been installed the tensioning jack is removed andany excess tendon is severed.

The tension in the tendon 10 urges the barrel 14 into abutment with theplate 38 which in turn is urged against the hanging wall 36 to providesupport thereto.

By varying the load at which the shear ring 20 fails, e.g. by varyingits thickness and/or the material of the barrel, the amount of preloadapplied to the tendon 10 can be varied. In this regard the Inventor hasfound that by making use of a barrel formed of steel such as EN9 andhaving a shear ring which is 2,5 mm thick a preload of about 15 KN canbe obtained. The filler material may be of a particular colour toindicate the pre-load which can be achieved by making use of aparticular barrel 14.

Reference is now made to FIG. 4 of the drawings, in which referencenumeral 50 refers generally to part of a setting device in accordancewith the invention and unless otherwise indicated, the same referencenumerals used above are used to designate similar parts. The settingdevice 50 will typically be in the form of a tensioning jack andincludes a nose 52 and a shear ring engaging member 54.

The nose 52 includes a body 56 and a collar 58 screw-threadedly mountedon the body 56. The body 56 and collar 58 define between them a radiallyinwardly open generally channel-shaped formation 60 within which theshear ring engaging member 54 is held captive.

The shear ring engaging member 54 is in the form of a discontinuous ringwhich, in its relaxed state, has an internal diameter which is largerthan the diameter of the trailing end of the body 18 and smaller thanthe diameter of the shear ring 20. The body 56 and the ring 54 areprovided with complementary surfaces 62, 64, respectively, which areconfigured such that when the shear ring engaging member 54 is urgedtowards the body 56 they cooperate to cause the shear ring engagingmember 54 to contract diametrically.

The setting device 50 is used in substantially the identical fashion tothat described above in order to pretension a tendon 10.

Hence, when use is made of the setting device 50, the shear ringengaging member 54 will be in its relaxed state in which its diameter islarger than the diameter of the trailing end of the body 18. The settingdevice 50 is positioned such that the ring 54 abuts against the shearring 20. As the nose 52 is displaced in the direction of arrow 66 thering 54 is urged towards the body 56 which causes it to contractdiametrically until its inner diameter is only slightly larger than theouter diameter of the trailing end of the body 18. Once thepredetermined load is reached, the shear ring 20 shears and the wedgearrangement 16 is blocked or set in the manner described above. When thebody 56 is displaced in a direction opposite to the direction of arrow66 the load applied by the body 56 on the ring 54 is reduced permittingthe ring to return to its relaxed state in which its diameter increasesthereby facilitating its removal from the barrel 14.

The Inventor believes that this arrangement will facilitate the removalof the setting device 50. In addition, if foreign matter becomes lodgedin the channel-shaped formation 60, the collar 58 can be removed so asto facilitate the removal of foreign matter. The inventor believes thatusing a barrel in accordance with the invention will facilitate thepre-stressing or pretensioning of a tendon and that the desiredpretension will be readily obtained.

What is claimed is:
 1. A barrel for use in pre-stressing a tendon, whichincludes a body having a leading end and a hole therethrough; and aspacing member in the form of a shear ring fast with the body andconfigured to transmit a compressive load applied by a setting member tothe body, the spacing member being designed to yield in the direction ofthe leading end of the body when a predetermined compressive load isapplied thereto.
 2. A barrel as claimed in claim 1, in which the body isgenerally circular cylindrical in shape, the hole extending axiallytherethrough and having a diameter which decreases over at least part ofits length towards a leading end of the body, the leading end of thebody being rounded, an annular recess is provided in the bodyimmediately adjacent the shear ring into which recess at least part ofthe material of the shear ring is displaced, in use, when the shear ringfails, the barrel including indicating means to indicate that thespacing member has failed and hence that a tendon, has been tensioned tothe required tension, the indicating means including a filler materialin the annular recess which filler material is at least partiallydisplaced when the shear ring fails and the filler material beingconfigured at least partially to absorb shock loads when the shear ringshears.
 3. A barrel as claimed in claim 1, in which the shear ring isformed integrally with the body.
 4. A barrel as claimed in claim 1, inwhich the shear ring is defined between a pair of axially spaced apartannular recesses in the body.
 5. A pre-stressing kit which includes abarrel as claimed in claim 1, inclusive; and a wedge arrangement, atleast part of which is receivable, in use, in the hole in the bodybetween the body of the barrel and a tendon extending therethrough.
 6. Apre-stressing kit which includes a barrel having a body through which ahole extends; a wedge arrangement, at least part of which is receivable,in use, in the hole in the body of the barrel between the body and atendon extending therethrough; and a spacing member formed integrallywith the body through which, in use, a compressive load applied by asetting member to the body of the barrel is transmitted until apredetermined load is reached at which stage the spacing member permitsthe setting member to apply a load to the wedge arrangement to set thewedge arrangement between the body of the barrel and the tendon tosecure the barrel in position on the tendon.
 7. A pre-stressing kit asclaimed in claim 6, in which the wedge arrangement includes a pluralityof wedge elements.
 8. A setting device for use in pre-stressing a tendonusing a barrel and wedge system, the barrel of which includes a body anda spacing member in the form of a shear ring fast with the body, thesetting device including a nose having a leading end and defining anannular radially inwardly open channel-shaped formation adjacent theleading end; and a shear ring engaging member which is in the form of adiscontinuous ring held captive in the channel-shaped formation andwhich is configured to engage and transmit a load to the shear ring ofthe barrel.
 9. A setting device as claimed in claim 8, in which the noseand the discontinuous ring include complementary formations configuredto cause the discontinuous ring to contract diametrically when thediscontinuous ring and nose are urged towards one another in use.
 10. Asetting device as claimed in claim 8, which is in the form of atensioning jack and includes a gripping formation for gripping a tendonand displacement means for displacing the gripping formation and thenose in opposite directions.
 11. A barrel for use in pre-stressing atendon which includes a body having a leading end and a holetherethrough through which a part of a tendon to be tensioned can pass;and a zone of weakness which is configured to yield in the direction ofthe leading end of the body when the body is subjected to apredetermined compressive load.
 12. A barrel for use in pre-stressing atendon which includes a unitary body formed from a single piece ofsteel, the body having a leading end and a hole therethrough throughwhich a part of a tendon to be tensioned can pass, part of the bodybeing connected to the remainder of the body by a zone of weaknessconfigured to yield in the direction of the leading end when the body issubjected to a predetermined compressive load so that said part of thebody is displaced relative to the remainder of the body when the body issubjected to said predetermined compressive load.
 13. A barrel isclaimed in claim 12, in which the two parts of the body areinterconnected by a zone of weakness which is designed to yield when thebody is subjected to said predetermined load.
 14. A barrel as claimed inclaim 13, in which the body is generally circular cylindrical in shape,the hole extending axially therethrough and having a diameter whichdecreases over at least part of its length towards a leading end of thebody which is rounded, the one part of the body being in the form of anannular shear ring, the zone of weakness being provided by acircumferential shear area at an inner diameter of the shear ring.
 15. Abarrel as claimed in claim 12, which includes indicating means toindicate when a said predetermined compressive load has been applied tothe body, the indicating means including an annular recess in the body,the width of which decreases when the predetermined compressive load isapplied to the body.
 16. In the pre-stressing of a tendon anchored atone end, by making use of a tensioning jack having a nose bearingagainst a tapered barrel around the other end of the tendon, there isprovided a method which includes steps of providing a spacing memberthrough which a load applied by the nose to the barrel is transmitted,the spacing member being designed to yield at a predetermined loadbetween the nose and the barrel; and using such yield to cause the noseto set a wedge arrangement in the barrel to grip the tendon.
 17. Amethod according to claim 16, in which the spacing member is designed toyield in the direction of the load to a predetermined extent.
 18. Amethod according to claim 16, in which there is provided an initialaxial clearance space between the nose and the wedge arrangement, theclearance space being arranged to be less than the extent to which thespacing member is designed to yield.
 19. A method as claimed in claim16, in which the spacing member is integral with the barrel via a zoneof weakness.
 20. A method as claimed in claim 19, in which the spacingmember is shaped and dimensioned to cooperate with the nose of the jackand includes an annular ring spaced from the rest of the barrel by anannular groove and in which a zone of weakness is provided by acircumferential shear area at the inner diameter of the annular ring;the axial length of the groove determining the extent to which thespacing member is designed to yield.
 21. A barrel for use inpre-stressing a tendon which includes a unitary body which is generallycircular cylindrical in shape and has a hole extending axiallytherethrough through which part of a tendon to be tensioned can pass,the hole having a diameter which decreases over at least part of itslength towards a leading end of the body, the leading end of the bodybeing rounded; and a zone of weakness which is configured to yield inthe direction of the leading end of the body when the body is subjectedto a predetermined compressive load.
 22. A barrel as claimed in claim21, which includes an annular groove in the body, the width of whichdecreases as the zone of weakness yields.
 23. A barrel as claimed inclaim 22, which includes an annular shear ring integral with the bodyadjacent to the annular groove, the zone of weakness being provided by acircumferential shear area at the inner diameter of the shear ring. 24.A barrel for use in pre-stressing a tendon which includes a unitary bodywhich is generally circular in shape and has a hole extending axiallytherethrough through which a part of a tendon to be tensioned can pass,the hole having a diameter which decreases over at least part of itslength towards a leading end of the body, the leading end of the bodybeing rounded; and indicating means integral with the body to indicatewhen a predetermined compressive load has been applied to the body. 25.A barrel as claimed in claim 24, in which the indicating means includesan annular recess in the body, the width of which decreases when thepredetermined compressive load is applied to the body.
 26. A barrel foruse in pre-stressing a tendon which includes a body having a holetherethrough through which a part of a tendon to be tensioned can pass;and an annular recess in an outer surface of the body, the width ofwhich decreases when the body is subjected to a predeterminedcompressive load.